1. Field of the Invention
This invention relates to NMR imaging. In a primary application it relates to rapidly acquiring the NMR data and producing high fidelity images relatively independent of the relaxation time.
2. Description of Prior Art
Existing magnetic resonance imaging systems use the 2DFT or spin warp system. This system is described, among other places, in a general tutorial paper "An Introduction to NMR Imaging From the Bloch Equation to the Imaging Equation," by W. S. Hinshaw and A. H. Lent in the Proceedings of the IEEE, Vol. 71, No. 3, March 1963. In this system a sequence of excitations are used, each providing a line in the spatial frequency domain, or k space. This is achieved by a variable area phase encoding gradient on one gradient axis, applied prior to readout which moves the k space scan to the desired line. Then, during readout, a fixed amplitude gradient is used to scan the selected line in k space. This provides a high quality image in a relatively long time since, for an n.times.n image, n separate scans are required, each of the order of one second.
A number of approaches have been taken to higher speed scans. Perhaps the earliest was the echo planar system described in a paper by P. Mansfield and I. L. Pykett in the Journal of Magnetic Resonace, Vol. 29, p. 355, 1978. In this method, following the excitation of a plane, a constant gradient as applied on one axis and a square wave on the other. This, in a single excitation, provides a rapid scan of k space. The time of the scan represents a compromise. If it is relatively short, the SNR of materials with long relaxation times will be compromised. If it is relatively long, materials with short T.sub.2 's will achieve only a partial scan of k space and be distorted.
Another approach to rapid imaging was presented by C. B. Ahn, J. H. Kim and Z. H. Cho entitled "High-Speed Spiral-Scan Echo Planar NMR Imaging" in the IEEE Trans. on Medical Imaging, Vol. MI-5, March 1986. Here a constant frequency spiral is used to scan k space. This also deals with the same compromise in scan time between SNR and distortion. However, since the scan is circularly symmetric, the distortion or blur function is symmetric. Also, the scan of k space has a non-uniform linear velocity, being greatest at the higher spatial frequencies. This results in a loss of high frequency response.
A general approach to high speed magnetic resonance imaging appears in European patent application No. 80104808.3, filed 8/14/80, publication No. 0-024-640 entitled "Moving Gradient Zeugmatography" invented by Richard S. Likes. A number of methods are shown for scanning the spatial frequency domain following a single excitation for rapid data acquisition. These include Lissajou patterns and rosettes generated by sinusoids, a spiral with varying initial phase and concentric circles. These reasonably fill the spatial frequency domain. However, materials with relatively short T.sub.2 relaxation times will produce distorted images, since only part of the spatial frequency domain will be scanned. If the scan is made short, to accommodate these materials, those with longer values of T.sub.2 will therefore have reduced SNR because of the reduced integration times.
Another general approach to rapid imaging is described in a paper by A. Macovski entitled "Volumetric NMR Imaging with Time-Varying Gradients," in Magnetic Resonance in Medicine, Vol. 2 February 1985. Here systems are shown using sinusoidal and other gradient waveforms. Methods are shown for correcting the impulse response and applying time-varying gradients to spectroscopy.